text:
Why we need 1 hour for cooling after 3 consecutive attempts for starting the APU?
As it's said here(APS-3200 training manual (pg 268)):

we need 1 hour for cooling after 3 consecutive attempts for starting the APU. But why?
why not after each attempt? and why 1 hour?
is it valid for all APUs?
What about the jet engines of aircrafts? is there something like that for them?

That is typical of any electrically started component.  During qualification testing of the APU, tests will be done to measure the heat build-up in the starter motor to determine how many start attempts it takes to get the motor's temperature to a critical level.  Other tests determine how long it takes to cool down enough to operate again, after it gets to that temperature.
In a nutshell, the limits are to protect the APU's starter/generator from heat damage if multiple start attempts are needed. Turboprop engines with battery powered starter/generators will have similar limitations.
Jet engines typically use bleed air driven turbine drive starters to start, and may have similar, if somewhat looser, limitations compared to an electrically started engine (to control the starter's gearbox oil temperature).
For example, on the CF34, you're allowed to crank for 90 seconds, and then there's a 10 second cool down after the 1st two attempts, and a 5 minute cool down after each subsequent attempt, up to 5 tries (after that, it's expected that you've given up and called for maintenance).  Other than that, no limitations.

In the Navy we used a ground "huffer" to supply external air for starting engines and still had similar limitations.

Yes the turbine starter itself still gets pretty hot.  On the  CF-34 the limitation is 10 second cool down after the first start, then 5 minutes between starts after. I added some context to my answer.

For the USN H-60 version of this APU, if you exceed these limits you can and will create fireballs that may leave flaming dribbling fuel down the side of your aircraft and scatter any civilian lookey-Lous.

The starter and start duty cycle limitations on APU's vary with the APU, and the installation (with the aircraft), and in some cases, with the operator.  The same limitations do not apply across the board to all APU's.
On the first start attempt, the start motor heats up,   It's given a minimal cooling time, and a second attempt allowed.  When this second attempt occurs, the start motor will not have cooled to its original temperature (where it was when the fist start attempt occurred).  The heat increase with the second start attempt, then, is cumulative.  In some aircraft, a third start attempt may be allowed, while in others it may not; if another start attempt occurs, the starter temperature increases even further.
If the APU attempts to light off and then fails, it will have an internal temperature increase which is not uniform, or "normalized" throughout the gas path in the APU.  Successive start attempts will worsen this condition.  Time is required for the temperature to decrease.
Remember that the APU, just like the main engines, is composed of many components which expand and contract at different rates.  The APU gets less airflow once the start has failed, than a wing mounted engine.  It has less ventilation, and less cooling.
Additionally, if fuel was admitted to the APU during the start process (depending on where the start attempt cut off), fuel may be pooled in burner cans, and could lead to a fire with successive starts; part of the time between starts is a draining period.
Another potential problem, depending on the power source used during the APU start, is ship battery depletion.  Successive starts can drain the aircraft batterie(s), and can lead to a large current draw from other sources, through the ship battery.  This can cause a battery overheat or even a potential fire, or may damage the battery.


text:
What is this part which is mounted on the wing of Embraer ERJ-145?
Take a look at the yellow parts which are mounted on the wing of an Embraer ERJ-145. What is it?

Two closer images:

From bottom:

They are called vortilons. They can induce a vortex to the upper surface of the wing at high angles of attack, which allows flying at higher angles of attack before stalling.

Additionally, four vortilons were installed on
the lower surface leading edge of the outboard wing panel. Their interaction with the wing sidewash at high angles of attack produce
strong vortices that are convected to the upper surface, where they
modify the pressure distribution and boundary layer development,
postponing flow separation and increasing maximum lift. Their
shape and position were defined using the 3D panel program. The
combined effect of the leading edge droop and vortilons allowed an
improved take-off and landing performance without resorting to
more complex variable geometry leading edge devices (such as
slats), for a small cruise performance penalty. Figure 22 shows a
schematic representation of the ERJ145 droop and vortilon.

(The evolution of the aerodynamic design tools and transport aircraft wings at Embraer)
They were first used on the Douglas DC9. The original patent shows the vortex on the upper side of the wing:

+1 for mentioning LE droop. This can be seen in the lowest picture of the question. Droop adds more to the maximum lift coefficient than vortilons do.

They are Vortilons, a type of vortex generator.  The placement of the vortilon low on the leading edge limits its vortex generation to high angles of attack (where the air meeting it is going over the top of the wing), and at low angles of attack, it's just a little fin sticking out, not creating too much drag (but not none).
They are used to improve aileron control at low speed, and/or stall behaviour where the effects aren't necessary at high speed.  They are definitely a "band-aid" added to fix a problem that came up during cert testing.  You generally don't add projecting bits like that unless you have to.

"You generally don't add projecting bits like that unless you have to" - winglets, tail-fins, huge rotating awacs radomes, pitot tubes, etc

A winglet is not rally a projecting bit.  It's a sail that provides thrust and weakens the circulation with its outwash so there is a net gain.  The other bits you mention, are there... because they have to be, for one reason or another.  If designers could measure dynamic pressure for airspeed without a pitot sticking out, they would.  If AWACs could do its job without the radome, it would. if Embraer could have tamed it's low speed aileron control problem without vortilons, at a manageable cost, it would have.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
How long can the stop in a stop-and-go be?
When a pilot is cleared for a stop and go, how long does this allow them to be stopped on the runway?
I'd like to know if there is a limit in regulations or at least guidance for this.

The AIM covers this briefly (bolded the important part) but effectively you cant sit there very long:

4−3−22. Option Approach
The “Cleared for the Option” procedure will permit an instructor,
  flight examiner or pilot the option to make a touch−and−go, low
  approach, missed approach, stop−and−go, or full stop landing. This
  procedure can be very beneficial in a training situation in that
  neither the student pilot nor examinee would know what maneuver would
  be accomplished. The pilot should make a request for this procedure
  passing the final approach fix inbound on an instrument approach or
  entering downwind for a VFR traffic pattern. After ATC approval of
  the option, the pilot should inform ATC as soon as possible of any
  delay on the runway during their stop-and-go or full stop landing.
  The advantages of this procedure as a training aid are that it enables
  an instructor or examiner to obtain the reaction of a trainee or
  examinee under changing conditions, the pilot would not have to
  discontinue an approach in the middle of the procedure due to student
  error or pilot proficiency requirements, and finally it allows more
  flexibility and economy in training programs. This procedure will only
  be used at those locations with an operational control tower and will
  be subject to ATC approval.

The way I read that is effectively you come to a stop and get going as soon as possible. If you intend on doing anything else you should inform ATC. I take this to be a time limit of, as soon as you stop, get going again.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
How to interpret/use "Fix Minimums" (at KALB)
This one stumped both myself and my CFI. Im specifically asking about the minimums at KALB IL/LOC 19 but a general answer is also appreciated.
The plate lists two distinct sets of minimums and we were confused about the S-LOC 19 minimums. The first set seems like the obvious regular ones and takes you to the labeled step down altitude. Then there is a second set for WESTS FIX MINIMUMS that are lower to the first set.
What is this second of set of minimums for and how can they be utilized?
It seems a bit odd that they reference an altitude below the step altitude. Presumably you fly 820 to 0.7 on the DME, but yet the WESTS FIX MINIMUMS seem to allow for lower than this.
(full plate for reference incase it changes)

Short answer, if you can identify WESTS, then you cross it at or above 820 and then go on down to your MDA of 620.
If you can't identify WESTS then your MDA is 820. Since the approach requires radar or DME, you could be in this situation & legal to fly the approach under radar control without DME onboard; the controller would tell you when you cross EYMEY (so you can descend from 2,000' to 1,600'), and then 6 miles later when you cross HAUKY and can start down to the 820' MEA, along with starting your timing for the missed approach. Both of the fixes are shown as a DME value or "RADAR" on the above chart; WESTS is only defined by DME, so without that equipment you don't get to use it and the attendant lower mins.


text:
Would formation of the three planes benefit from increased distance of the ground effect?
I know that the ground effect quickly diminishes as altitude above the ground increases, and it is roughly half of the wing span. Hence it can be used to fly more economically over water, but over ground normally would not be high enough.
I have envisioned two larger airplanes "helping" the smaller one in between by flying next to it as shown in the diagram. By idea, wings of the all three planes would form much longer "superwing" with the ground effect equal to the complete wingspan of the larger plane plus half of the wingspan of the smaller plane in center. This would be much higher than the small plane could do alone (half of its wingspan), maybe making possible to use the ground effect also over ground. The small airplane would then need less effort to fly, if it is, for instance, low on fuel.
Of course, the obvious problem is that the wings cannot "join", it must be some reasonable gap there to avoid the collision.
Would the central plane flying in formation as shown in the schema experience a stronger ground effect than flying alone?

Why not join the wings?  :-)

I would think not; the ground effect comes from the wing pushing air down, which would not help the plane in the middle. If anything, I think the plane in the middle would be impacted by the wingtip vortices created by the larger planes, once out of ground effect (one wingspan above the ground) if the smaller plane did not stay ahead of the larger planes.
http://www.pilotfriend.com/training/flight_training/aero/wng_vort.htm
A position slightly behind, drafting like birds in flight do when they fly in V formation, might help.
https://www/ciencemag.org/news/2014/01/why-birds-fly-v-formation


text:
What is this intake bottom the wing of C-5 and why is it placed there?
There are two intakes bottom the wings of C-5. What are those and why are they placed there?

This C-5 cutaway indicates it's a "Heat Exchanger Inlet (76)" or "Ground Cooling Fan (78)"
EDIT: Upon closer inspection of this higher-res image, it may be the intake for the Ground Cooling Fan.


text:
What kind of test is going on in this image?
Some times we see images of jet engines on the net that have strange colors. For example take a look at the bellow image. What the operator is doing? Why the color of blades are white with red on its borders? why are they numbered? why is the color of spinner yellow?

Source of image

This is most likely some kind of a test where high speed photography (as in 5,000 - 10,000 frames/second) will be used to capture the detailed motion of the fan blades.
Types of tests that might require this type of video include fan blade out testing, bird strike testing, and icing / hail testing.  There may be others but those are the most common.

Why the color of blades are white

In order to get 10,000 frames per second you need a lot of light. There will be many high powered lights shining on the engine.  You don't want harsh reflections from the lights, so you need a matte finish, the flatter the better.  The normal blade finish is way too shiny. So they are painted.

with red on its borders?

just for contrast, so you can very clearly tell in video where the blade stops and the fan case starts.  The exact color is arbitrary, and different OEMs have different traditions.  You are just looking for a set of colors are very different from one another.

why are they numbered?

So in the high speed video, you can see what happens to what blade.  For example, in a hail test, you might observe that a giant chunk of hail hit blade #15.  You could then go inspect blade #15 afterwards and see if there is any damage.

why is the color of spinner yellow?

Same as the red border. Just for contrast, so you can see where the blades stop and the spinner starts.

What the operator is doing?

I don't know exactly. Obviously some kind of assembly or disassembly procedure.  You can tell in that photo that the inlet is not attached, he's right up against the plane of the fan blades.  So it's not a fully assembled engine on the airplane or on a test stand.  It's clearly in the factory somewhere.  But what exactly he's doing I don't know.  Honestly could just have been "hey we need some publicity photos for Aviation Week, pretend to do something interesting with the engine".

White & red combo is good, as it has high contrast in color, greyscale and black and white formats.

The blades may be part of a test fixture.
The uncropped image is from a 2018 report in the Telegraph, which  includes another photo captioned

Rolls-Royce's new Pearl engine for private jets undergoes tests
Credit: Rolls-Royce

Labeling blades for tests is very common, even for those run at full throttle. If the label is for example drawn on the blade with a marker using a stencil, the added weight will be insignificant. For example see https://youtu.be/736O4Hz4Nk4  1:24 onwards.


text:
What's this stick-like thing on the glass of C-130J?
Just was watching this video:
C-130J Shadow Formation Landing
You can see this stick-like thing on the glass of the aircraft.

What's it? a sensor?

It can be for window heating, but I am just guessing

Just watched the video. That's a bizarre camera angle - I thought he was crabbed against a massive crosswind, but it's actually the angle of the camera that makes it look that way. Either that, or there's a lot of curvature of glass on the nose of a Herc so that it just appears that we're looking out the side.

@FreeMan, the camera is obviously looking through the side window (it is behind captain's left shoulder), a little to the left, and is tilted to the right so the horizon does not appear level. Looks like just shoddy camera placement.

It's part of the windshield anti-icing system, and is the same as on the Navy's C-130T. From the NATOPS flight manual for the T model, page 2-150:

The three windshields, the two windows on each side of the windshields, and the two lower windows in front of the pilot are NESA-type. These panels are heated by applying unregulated ac power from the left-hand ac bus to a resistance material between the layers of glass. The ac power is applied by automatic dc control systems that cycle to maintain window temperature within specific limits. A center windshield system controls heating of the three center windshields, and a side and lower system controls heating of the side and lower windows. The two systems are identical except for the amount of total ac power provided.


text:
How the EFD system of F18e shows the fuel in tanks of the fighter?
I was seeing this image:

How the EFD system of F-18E/F shows the fuel in the tanks of the fighter? I mean it's very low. e.g. 7.2 lb? Is it rounded?

Fuel qty is displayed in 1000s of pounds. The Fuel Qty Indication System measures fuel quantities from the individual internal and external tanks and displays them on the Engine Fuel Display in 1000s of pounds, corrected for ambient temperature.
The EFS above indicates the jet has 7,200 lbs of internal fuel onboard with a preset bingo fuel state of 4,500 lbs.


text:
What does mean PPM unit and why it is the unit of flow rate for the ASU(Air start unit)?
I was seeing the specifications of several ASU(Air start unit)s and found out that makers of such machines provide the flow rate of the ASU in PPM. Why? why not Volumetric flow rate?
Example:
TS200 Air Start Unit

PPM stands for Pounds Per Minute (not to be confused with Parts Per Million, which also uses PPM), which is a unit of mass flow rate.
The specification gives the mass flow rate rather than the volumetric flow rate because the mass of air going in and out of the ASU is the same, where the volume going in and out is different because of the compression.

Perfect answer! thank you!


text:
Why do the engine of MiG-31 spit out flame when going for afterburner?
I was seeing this image. Always we can see an abnormal flame spit out of the engine when the pilot goes for afterburner. Why?

It's totally different from F-111's fuel dump flame. Probably the afterburner is simply too cold and the fuel doesn't burn very well.

@user3528438: I wasn't also that sure. Can you elaborate an answer from your comment?

Do you mean always in the case of MiG-31, or always regardless of make/model?

@Jpe61 I just have seen in the case of MiG-31.

@user3528438 Ok but, why does it happen on the below portion of the flame?

The thumbnail of this video is a little misleading (it isn't from the same video), but show the same phenomenon.  The video has a different angle from a little further behind (also at a completely different time and place I think since the thumbnail has snow), but it looks like there's just some kind of unstable transients as the afterburners engage.

It's possible that the fuel flow rate as the afterburner engages is simply too high for the airflow for a couple seconds, so there's fuel pooling and combusting as a pool of flame at the bottom of the afterburner section of the nozzle instead of getting properly mixed and burning in the exhaust stream.
This other video also seems to show the same thing, but only visible in the right engine, and it's harder to see.  I think that supports it being some kind of transient that doesn't always occur.  Watching it frame-by-frame it might also be that the various nozzles of the afterburner (which are typically a ring of fuel nozzles around the afterburner duct) just aren't quite in-balance; I thought initially that the left engine had a flame front starting from the center (i.e. in the exhaust stream) but it's off to the left side.

I haven't been able to find a video looking up the nozzle of a MiG-31 igniting its afterburner in flight; I think that would be a helpful test of my guess.
(My bona fides, since there's some speculation in this answer:  an aerospace engineering degree and a pilot's license, though I've never worked directly on jets.)

You can embed YouTube videos, I believe.

@RodrigodeAzevedo the embed won't show a specific frame, though; and, as I say, the first video has a misleading thumbnail, which is what would embed.  ...also, I always forget that you can do that, but this time I stand by not embedding them.


text:
What does this logo on prototype aircrafts or cars for test mean?
Sometimes on prototype cars or aircrafts we see a logo just like BMW logo. e.g.

Why? What does it mean? What is it for?

Does this answer your question? Checkerboard painted on fuselage of Boeing test aircraft Or this? What is the purpose of the black and white stickers on test aircraft?

This is simply no logo, but a reference that is both highly visible and can be measured very precisely in photos and videos - this giving a valid reference point for all measurements and analysis.


text:
What are those half-vanes in the engine inlet of a 707-328?
I just saw this:

Source
They look like IGVs but I'm not sure. What could those be? Do you know the engine?

Another view from this page

You have a very interesting find there! It is in fact not an engine from a B707, but a Kuznetsov NK-8 from Russia.
It was used in Ilyushin Il-62 and Tupolev Tu-154 aircraft.
As for the intermediate vanes, it is my understanding that their purpose is no more special, than to just stabilize the airflow entering the bypass section of the engine.

(Picture taken from the site from the first link in this answer)

Yes. This (both the stub and full vanes) is called IGV (Inlet Guide Vanes). Evidently, stabilising the flow around the tips was considered more important.

Older jet engines probably were more critical about the flow characteristics, adding those half vanes keeps the distance between vanes more equal.


text:
Why do new helicopters have rotor blades with swept tip?
Many new helicopers have rotor blades with swept tip. Why? Acoustic? e.g.

Very related: Why don't helicopter blades look like other propellers?

Thank you guys, just voted to close. just keep in mind that these two questions could also be helpful: https://aviation.stackexchange.com/questions/33573/does-the-main-helicopter-rotor-blade-shape-affect-noise and https://aviation.stackexchange.com/questions/14006/why-do-propeller-blades-not-have-winglets

Helicopter rotor design is, among many other things, a balancing act of retreating blade stall and transonic effects on the advancing blade.
Helicopter blade tip devices usually serve to reduce transonic effects on the advancing blade. The Ka-50's blade tip is rather simple (only swept) as the coaxial design is affected less from these transonic effects, as they happen on both sides. For the same reason, retreating blade stall is also a lesser problem, so the designer can get away with a slower blade tip speed.
Helicopters with a single main rotor don't have this luxury, so many of them have complex blade tip devices with sweep, larger chord and sometimes different airfoils. I believe a good example is the Westland (now merged, AgustaWestland) Lynx.

Blade leading edge is top of picture. Sweep, increased chord and droop on end are evident.
For my Bachelors thesis, I was looking for blade tips with the highest drag divergence Mach number, the Lynx blade tip's was M 0,91. I believe this is an important reason why the Lynx is the fastest conventional helicopter.

Among other thing for the KA-50 in the picture is to-

The rotor blade double-swept-tip geometry can effectively improve the aerodynamic and aeroacoustic behaviors of a rotor

Source:
Open Access Aeromechanical Stability of a Bearingless Rotor Helicopter with Double-Swept Blades
A review of helicopter rotor blade tip shapes

A major part of this shape/design is to prevent the tips of the blades from going Supersonic. Allowing the helicopter to fly faster in a straight line. Your answer does cover this, but I think this is the main reason.

The cutaway drawing depicts a KA-50, not AH-64

Thanks @DeepSpace, fixed


text:
What are these holes on the intake of Progress D-18T which are mounted on AN-225?
I was seeing a Progress D-18T of AN-225 and saw these holes on the intake of them. What are they for?

My guess is exhaust holes for cowl anti-ice bleed.

@JohnK I was going to say the same thing. You beat me to it. :)


text:
Why does J79 have speed more than 100%?
In the maintenence manual of J79 saw this table:

As you can see, there is speed more than 100% and it looks that the engine can handle it. Can't it?
One more thing, days ago, watched this movie: flight. The co-pilote had conversation with the captain about "engine overspeed" when the aircraft was in a dive. Is this related to this speed which is more than 100%? I mean maybe the factory choose a speed lower than the max. speed to prevent engine overspeed when e.g. the aircraft is in the dive.

@ymb1 Ok, but in many cases it's very hard to find the characteristics and specifications of other engines or you don't know how to google your question to find the answer. I think you have been here for a big period of time. For instance I tried to find the manual of JT8D or CFM56 but found nothing.

@ymb1 Ok, thank you.

@ymb1 Thanks! Obviously I missed that consensus :-)

Various models of the J-79 family of engines have been used on a wide variety of aircraft manufactured in different countries. More than 17,000 of the engines were produced over a period of 30 years and used on aircraft such as the F-104 Starfighter, B-58 Hustler, F-4 Phantom II, A-5 Vigilante, IAI Kfir and SSM-N-9 Regulus II supersonic cruise missile. In some aircraft, like the B-58 Hustler, the pilot could choose to override the maximum power throttle stop and squeeze a little more RPM out of the engines.


text:
What's this outlet on the wing of Eurofighter Typhoon EF2000 for?
I was seeing this image and saw this outlet.

It's not the outlet of an APU, is it?

Duplicate question confirms, it IS the exhaust for the APU.


text:
What's the name of this part of combustion chamber and what is it for?
There are two "excrescences" Near the end of diffuser of some combustion chambers. e.g. CFM56 and F101
What's the name of this part of combustion chamber and what is it for?

I don't known if it has a particular name, this is just the rear end of the diffuser, but it seems for maintaining the flow continuity as long as possible, orienting air onto the swirl nozzles and limiting air entering the combustor through the inner liner cooling and dilution holes.

On depictions of the CFM56 combustion chamber, that's part of the cross-section depiction of the diffuser.


text:
What are these two circular areas on the fuslage of Fokker 100?
There are two circular areas on the fuslage of Fokker 100 which are shown in the bellow image. What are those? Why are the above these two areas written "do not paint"?

After googling "static port plates", found these two also https://aviation.stackexchange.com/questions/56625/what-are-the-static-ports-in-a-boeing-767 and https://aviation.stackexchange.com/questions/17440/how-does-a-pitot-plate-work hope help


text:
Is this part of the engine intake an aerodynamic component?
Just was seeing this image:

And found out there is a component that split up the intake into two parts. Is this part of the engine intake an aerodynamic component? if so, how does it work?
Edit: I think the plane is a General Atomics MQ-9 Reaper.


text:
What's the name of this component of the turbofan engines?
Almost many of turbofan engines that I've seen so far nowadays had a part like a small wing. what do you call them? here is some pictures of the thing I'm asking question about:

The actual nomenclature would be a "strake" or "chine".

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Why does intake of turbofan engine have angle of attack?
As you can see the title of the question, intake of turbofan engine has an angle of attack. why? see:

Small detail: What you show in blue is not an angle of attack.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
What's the thrust coefficient for a convergent nozzle?
As you know, the Ideal Thrust Coefficient of convergent-divergent nozzle is calculated using this equation:

But what about a convegent nozzle? Is there any equation to calculate the Ideal Thrust Coefficient for a convergent nozzle?
Image from: this link

It’s exactly the same equation, but now the throat is at the exit. So, Ae = A*, and so the areas disappear from the last term.

What about Pe? are you sure about your answer? Do you know how this equation is drived?

@Roh. Although the exit is now at the throat, the exit is still the exit. So, Pe is still Pe. Yes, I did look at the derivation, given by your link.(Thanks fir that, it was very useful). A convergent- divergent nozzle is a more complex geometry. So, a formula for that can be reduced to the simpler situation of a convergent only nozzle, but not the other way around.

One must be careful when working with those coefficients, they account for many losses that make differ the ideal thrust from de real one (components of exhaust velocity in a radial direction, presence of boundary layer friction, mass flow leakage, etc.), and those losses may vary depending on the author.
I don't know how this equation was derived, but when I studied jet engines a factor in $C_f$ expression was from a empirical basis, so maybe you cannot use only your math skills to get there.
Respect your question, I think the equation can be used directly considering that $A_e=A^*$ if the nozzle is choked ($M_e=1$) and using the proper expression for $p_e$.
Note: anyway, for a rocket, the relation between total pressure at the entry of the nozzle and the ambient pressure is so great that a con-di nozzle is always used (I've never seen a rocket fitted with a convergent one).


text:
Do thrust reversers make more noise than engine at same RPM?
I'm talking about general aviation jet engines here. I live near an airport and I rarely listen engine noise on takeoff but always hear one during landing.
Would a jet engine at the same RPM make more noise when using thrust reverser than not using one?

Yes. Most of the noise you hear from aircraft is not mechanical noise from engines but disturbances in airflow around the airframe and engine outlets and inlets.
The airflow from reversing jet engine is much more turbulent than when the engine is forward pushing and thus more noisy.


text:
Why are the engines not tucked underneath the wings of airliners?
If you look at the engines of an A350 and other similar aircraft, you will see that their engines stick out ahead of the wing. Why? Why aren't they tucked under the wing?
(Source: Airbus)

Also Why do we use pylons to mount the engines on jetliners?


text:
Why are the intakes of some fighters not attached to the fuselage?
Look at the photo below. As you can see the intake isn't attached to the fuselage. Why? Other examples include the F-16, F-22, and F/A-18 Hornet.
(F/A-18E Super Hornet Walk Around)

Actually more are not attached than attached, e.g. DSI design. https://en.wikipedia.org/wiki/Splitter_plate_(aeronautics) https://en.wikipedia.org/wiki/Diverterless_supersonic_inlet .

And if you get a chance to look closely at an FA18, e.g. the F18 HARV at Virginia Air and Space Center, you can see in the gap, there is a smaller splitter under the splitter, and holes on the fuselage side that sucks away the boundary layer. Pretty interesting.

The problem which this gap is solving has to do with boundary layer air as @ymb1 suggested but performance is not the only complication.
Boundary layer is not only slower, in some regimes of flight it can become highly turbulent compared to the air that is offset from the aircraft's surface and is freely ingested by the aircraft's intakes. Mixing the two makes the resulting flow unstable and thus combustion inefficient and even choked.
At supersonic speeds these issues are even more severe.
Some designs go without the gap and use 'gauzing panels' which remove the boundary layer.
Others - use hump-like structure on the fuselage of the aircraft right in front of the inlet to divert the boundary layer air away. This design was tested on the F-16 you mentioned, but is already a key feature on F-35.
See the related article.


text:
Where does this integral for a 'aerospike nozzle' come from?
Today I was reading about the aerospike nozzle and found this image:

(from section 10.3.1 of this lecture)
This image tells us how to measure the thrust of this kind of nozzle. I can understand all of it except this part:

Where does this integral come from? How do they derive it?

Where is the image from? Besides citing your source being a common courtesy, it might make answering the question easier.

@JanHudec here is a link to the source of the image: http://web.csulb.edu/colleges/coe/ae/engr370i/ch10/sect_3-1/

On this site question (and answers) are editable and additional information should be edited in. I've added the citation for you now.

I expect you know how a conventional rocket nozzle works: It expands the exhaust gas, and the pressure inside helps to increase thrust, pushing the nozzle forward.
Now turn the conventional nozzle inside out. The expansion happens on its outside, but there is still a pressure acting on a backward-facing surface. The graph doesn't explain it, but the ramp area $\text{A}_{\text{Ramp}}$ is the projection of the nozzle area in flight direction. The ramp pressure $\text{p}_{\text{Ramp}}$ is the pressure acting along the nozzle surface, and it must be reduced by ambient pressure $\text{p}_{\infty}$ in order to arrive at the correct thrust. The integration is needed because $\text{p}_{\text{Ramp}}$ varies over the nozzle area.

But the integration is proportion to A not Pramp.why?

@Roh: The pressure is integrated over the area. Both influence the result. How else should this be done?


text:
What is the MEL (minimum equipment list)?
What is a minimum equipment list for an aircraft? How are MEL and MMEL related? Who creates the list, and how do they decide what is on it?

US?  Part 121, 135 or 91, or 137?

As mentioned here:

A minimum equipment list is a list of equipment that must be installed
  and operable for the aircraft to be considered airworthy. It is
  aircraft-specific and spells out which pieces of equipment may be
  inoperable while maintaining airworthiness. If something is found to
  be inoperative, the pilot goes to the MEL, finds the entry for that
  item, and determines if the airplane must be grounded until that piece
  of equipment is fixed.

A Master minimum equipment list is:

An MEL for a specific aircraft originates from a master minimum
  equipment list (MMEL). The MMEL is a list of all equipment on an
  aircraft type; it details which equipment is allowed to be inoperative
  without grounding the aircraft. (Think of a MMEL as a general, broader
  MEL.)

FAA publishes MMELs by manufactures.

The MMEL (Master Minimum Equipment List) is generic across a type of aircraft.  e.g. Cessna Citation CE-500, CE-550, CE-560.  A single MMEL is published by Cessna to cover all of these models from the factory (Master).
A MMEL can not, and does not, take into account differences that may occur during upgrades and modifications, nor differences that may be present within specific airframe serial numbers.
A MEL (Minimum Equipment List) is a 'custom' document generated and approved for a specific airframe serial number, or a group of aircraft that are identical (fleet).  In a fleet situation of like type airframes with minor differences, a MEL can spell out which serial numbers are applicable for a given item.
Once a MEL is approved, it becomes a 'bible' for dispatch or no-dispatch.
A MMEL is not enforceable, as it is not minded by a LOA, but is more guidance in determining whether dispatch is recommended.  A MEL can be more restrictive than a MMEL, but not less restrictive.  By the definition of 'airworthy'; therefore, the MEL may ground a plane for certain operations, but does not make the aircraft 'unairworthy' for other operations.  e.g. An operation operating an aircraft under a Part 135 certificate with a more restrictive MEL approved for the 135 may ground the plane for 135 operations, yet the owner may operate the same airframe under Part 91 using their MMEL if allowed.
The FAA would love all jets to adhere to at least a MMEL, and push operators to get a MEL approved through the LOA (Letter of Authorization) process.

The MEL is NOT a list of equipment that must be installed and operable for the aircraft to be considered airworthy. The correct definition is:
The MEL is a list of aircraft equipment that can be inoperable PRIOR to dispatch. MEL items are only applicable before your flight is dispatched. Once you are in the air you need to consult the aircraft QRH.

Hello Antonio, welcome to Avitation.SE. Can you possibly provide citation for your statements above? Please also see our Help Center for a short description on how a good answer looks like!

I disagree that the phrasing is that bad. The word "airworthy" is defined as "fit or safe for flying", which is the same thing as you are saying in your second paragraph, so there is nothing really wrong with the definition. "installed and operable" also implies before flight. If the stuff failed during flight to push it below MEL, it's questionable if it would be considered 'airworthy' any longer (i.e. safe to fly), independent of what the QRH says.


text:
What is the authority of the Air Boss at an airshow?
Regular ATC operates under regulations like FAA Order 7110.65 when they control traffic. At an air show, there is typically an "air boss" that manages aircraft within the air show area.
Is the air boss required to have the same training and certification as a regular controller?
Does the air boss operate under the same regulations as regular controllers?
Does the air boss have the same authority as regular controllers?

Air bosses who orchestrate air shows do not receive special training or certification . Infact FAA also lacks the resources to inspect all aircraft that participate in air shows
FAA doesn't have certification standards for air bosses . Waivers for customary flight standards are provided to air shows, which then hire people to direct planes in flight.
There is no training in place right now for an air boss
http://www.rgj.com/story/news/2014/03/23/ntsb-hearing-addresses-distance-between-race-course-and-spectators-other-issues/6587627/
However,  typically former air-traffic controllers with a great deal of aviation experience are hired as Air Bosses at Air Shows .
The air boss implements the standards on the FAA waiver
Firms like ICAS provide Air Boss training


text:
Does go-around require more power than take-off?
The complete version of Buran spacecraft would have had two engines that were not sufficient for take-off under own power.
Is it possible to say that if the aircraft is not capable of taking off under own power (not sufficient), it should not be capable of going around (assuming weight and other circumstances are the same)?

Depends on if the aircraft was capable of (any) climb rate at the airport density altitude at the time you would be attempting to go around. Since you are already flying, getting up to speed isn't the issue, the issue is gaining altitude without stalling (or hitting things when you are climbing too slow).

@mins The in-atmosphere test version had 2 jet engines installed. I don't believe the orbiter version would ever have the jet engines installed, I think that is where the OP may be confused. This (and the other Buran question) are borderline better on Space.SE.

@RonBeyer, according to wikipedia the in-atmosphere test version had four engines and was able to take off under it's own power.

@mins, Buran does not have a “main tank”, but a first stage rocket with engines mounted on that rocket. The orbiter has two engines that serve as second stage, although with rather small Δv of 66.7 m/s, and deorbit engines, with fuel inside the orbiter itself. Of course they would be out of fuel, or almost out, by the time of landing.

@JanHudec: I didn't know, indeed this makes my comment ridiculous. Thanks for correcting,

@mins, then I found there was also idea (never realized) to keep two of those four jet engines from the test aircraft to the actual orbiter. That would not be enough for going around, so it would be of very limited usefulness.

Please retract the landing flaps!


text:
Has putting car tires over wings ever been used to protect the aircraft from airborne attack?
Recently Ukrainian bloggers have published videos, where Russians have placed the car tires on the wings of they parked strategic bombers (Tu-95), most likely to provide some protection against drones (source, Twitter/X):

What you are looking at is a satellite image featuring a TU-95
strategic bomber covered with car tires. According to them, this
should protect strategic bombers from drones

The post depicts this as a desperate action unlikely to work, but is it? Has this way of protection ever been tried, and if so, with which success?

Can you post the picture here as well? Not everybody has a Twitter account

This is not my Twitter post. I am not sure if I have rights to do this. Simply car tires lying on the side, on the upper surface of the wings of TU-95, single layer.

This sounds like an attempt at disruptive coloration.
The use of car tires likely has no significance beyond the fact that military bases tend to have a lot of tires in them which can be repurposed in a hurry.
Certainly camouflages have been used with some success many times.  Probably camouflages that attempt to protect a strategic bomber from drones are untested in war, since this is the first war where drones attacked strategic bombers.

May at least confuse the neural network of the drone computer vision, if present. It is likely trained to recognize wings without tires.

@h22 do you know if the drones fly by themselves only, or if they are human-operated/supervised? If it's the latter, then the effects are much more limited.

@ROIMaison most of the rockets likely do not need a remote pilot, so I do not think that a battle drone requires this kind of assistance by design. Fully autonomous flights are not allowed in civil environment, likely no one cares in the battlefield.


text:
Why is the nose of the Mig-21 shaped so differently compared to other fighter aircraft?
If you look at this picture below, you will notice that the nose portion of the Mig-21 has a unique conical shape which is completely different from the nose of the F-16.
(Source)
(Source)
So what explains the conical design of the nose of the Mig-21?

A lot of aircraft had that nose/intake design

See https://en.wikipedia.org/wiki/Inlet_cone which has information on the use of inlet cones.

This video on YouTube shows how the SR-71 engine worked. In particular, it has an animation and explanation of the reason for the inlet cone, which is to reduce the supersonic airflow so that the engine can use it. I was surprised that the inlet cone actually moves forward/backward within the intake, dynamically changing the geometry to match conditions.

Is it me, or does the top picture appear not to have an inlet?

Please provide proper attribution of the photo as required by SE

@Mikey 4 years on :-). It's you. The inlet is the circumferential ring around the central cone. You get a much better feel for it in this Wikipedia image.

The nose design of aircraft, like any other part, is a result of optimization in response to a number of factors. The Mikoyan-Gurevich MiG-21 'Fishbed' is a supersonic, second/third generation Soviet fighter/interceptor.
The first jet aircraft to enter serice during the WWII, the Messerschmitt Me 262 'Schwalbe' and Gloster Meteor had twin engines in their wings. In these cases, the nose was designed to carry the cannons, which were the primary armament at the time.

"Messerschmitt Me 262A at the National Museum of the USAF" by USAF museum - http://www.nationalmuseum.af.mil/shared/media/photodb/photos/040820-F-1234P-081.jpg. Licensed under Public Domain via Commons.
"F86Sabre" by Kowloonese at the English language Wikipedia is licensed under CC BY-SA 3.0
The first generation jet fighters, like the F-86 and Mig-15 were subsonic and had engines buried into the fuselage to reduce drag. The nose, again carried the cannons (in case of Mig-25, in the lower fuselage).
However, the second generation jet aircraft were the first operational jets to go supersonic. The Mig-21 was designed in response to a Kremlin specification, which called for Mach 2 level speed (at altitude), while carrying a simple radar and option for future air-to-air missiles.
As the jet engines require airflow to be subsonic at entry, the flow has to be decelerated before reaching the engine. Early in the design (from Ye-50-3), Mig decided to use a conical centerbody for this purpose, as it was decided that the aircraft would be single engined with engine in the fuselage (the engine was supposed to be the Mikulin AM-9B, however, the aircraft ended up using Tumansky R-11).
This nose design decelerated the flow using two oblique shocks and a normal shock, giving improved pressure recovery at high supersonic Mach numbers. The center-body (cone) moved axially according to the Mach number. Also, the center body housed the radar and associated equipment.
The first generation Mig 21s only had a range finder, and the intake diameter had a smaller cone. The following image shows the range finder used in early models of Mig 21.

"Mig21F13web" by Newresid stephanelhernault@yahoo.fr - Own work work vector drawig travail personnel dessinn vectoriel. Licensed under CC BY-SA 3.0 via Commons. (Image cropped)
"MiG-21F-13 radar Keski-Suomen ilmailumuseo" by MKFI - Own work. Licensed under Public Domain via Wikimedia Commons.
The photo you've shown are from the later versions (Mig-21 bis from IAF), which had the radars from the "Sapphire" family. To fit the radar antenna into the cone, the cone had to be made bigger, therefore the whole front fuselage had to be redesigned, the diameter of intake got more than 200 mm higher, and the cone itself got longer, wider and moved forwards. The following figure shows the enlarged nose cone section housing the radar. Interestingly, the nose cone seems to be made out of (metal tipped) wood to prevent interference.

"Mig21fishbedHfamilyweb" by Newresid stephanelhernault@yahoo.fr - Own work work vector drawing travail personnel dessin vectoriel. Licensed under CC BY-SA 3.0 via Commons. (Image cropped)
Source: toad-design.com
A number of aircraft from that time had conical body inlets, like the British English Electric Lightning.

"Lightning.inflight.arp.750pix" by Arpingstone - Own work. Licensed under Public Domain via Commons.
The F-16, on the other hand, has a simple normal shock diffuser. By the time of F-16, the radars had become bigger and more important (AAMs have become the primary combat weapon, replacing guns), and needed more space in the nose. As a result, the inlets of most of the aircraft had moved to the sides/downwards. The following figure shows the AN/APG-68 radar in F-16.

Source: defenseindustrydaily.com

A lot of early jet aircraft had the intake in the nose.  Here's a few more examples:

The cone in the front is required because the Mig-21 is a supersonic aircraft.  The cone breaks the shockwave so that the inlet air is sub-sonic.  The SR-71 is a very different type of aircraft but you can see that its engines have a very similar style of inlet.

This doesn't answer the question - what is the design philosophy behind the shaping of the nose of the Mig-21

@Simon - but it does; the last statement says the cone breaks the shockwave of supersonic air, so the air entering the inlet is subsonic. Later designs moved the intakes far enough back on the body that the nose cone of the aircraft did the same job.

Going off topic, but is it the case that nose intakes are no longer used because the nose is a good place for a radar?

@Crosbie That's a great question -- you should ask it as a question!

Please provide proper attribution of the photos as required by SE

@Crosbie I agree with David, you should ask as I want to know too. Although I think it is because while it is a good place for inlet, it results in bad placement for the engine and exhaust.

some missile also have it http://pib.nic.in/photo//2007/Jan/l2007012612196.jpg

@Crosbie The question related to the radar has been answered by aeroalias. Please check his answer.

@Simon the question asked why it's different. The answer is that it isn't :)

The design philosophy which led away from the nose intake was to fit a powerful radar to the aircraft, which in turn was a consequence of switching from guns to air-to-air missiles as the primary armament of fighter aircraft.
Most designs of the late Forties and early Fifties did not consider powerful radars in individual aircraft, but were based on ground radars and radio communication to direct the aircraft towards their opponents. Much of that strategy was an extrapolation from World War II, where large groups of bombers had to be intercepted.
In the Fifties, the advances in electronics and rocket propulsion made air-to-air missiles possible, and gradually the fighter aircraft found themselves in the role of ground stations, scanning a wide space deeply and directing their missiles to individual targets, just like ground radars had done with fighters in the closing years of World War II. Radar range and performance required a large dish in the nose of the aircraft, and the intake had to move to the sides. In parallel, the increased work load for the individual aircraft resulted in two-seater designs, where the radar system was operated by a dedicated back-seater.
Note that the MiG-21 or the BAC Lightning did have a radar in the nose cone, but this was a very small radar for local orientation and not suitable for detecting targets more than 100 km away and directing guided weapons.
As to why the intake was shaped as it is, please read this answer here on Stack Exchange on the design of supersonic intakes.

North American F-100 Super Sabre (left, picture source). Just like the MiG-21, this was designed to be led from the ground and engage the enemy in visual conditions.
Convair F-106 Delta Dart (right, picture source). This was the first fighter designed to use missiles as a primary weapon. Its radar system was extremely complex, combining on-board with ground assets, for all-weather operation using missiles over long distance, including a nuclear-tipped version for use against big bomber formations.

Just so you don't think that Mikoyan would not try to integrate bigger radars (picture source): The Ye-8 prototype changed the intake to an F-16 style belly intake to make room in the nose for a Sapfir 21 radar, which enabled the MiG-21 to engage targets in all weather conditions and at night. First flight was in April of 1962, but no production followed.

Radar of the Phantom II (right, picture source)
McDonnell-Douglas F-4 Phantom II (left, picture source). Just a decade after the F-100, the nose was designed around a 81cm radar dish and the aircraft gained a dedicated radar operator to engage enemy aircraft without ground support and in all weather conditions. Now, it can be argued that its side intakes were a consequence of using two engines, but the F-106 used a single engine and still had similar side intakes.
Advances in computer control reduce the workload and the size of the aircraft, so the F-16 can be operated by a single person and still carries a radar powerful enough for independent all-weather operation.

It is the intake of the jet engine which gives it that shape, a good number of earlier jet fighter planes did have that configuration. The pointed part is the shock cone for when the plane flies supersonic so that the shock would not mess with the aerodynamics of the plane especially inside the engine.
Image by USAF

A lot of supersonic aircraft of the era had the same kind of intake, the English Electic Lightning off the top of my head:

EE Lightning F6 'XS904 / BQ' by Alan Wilson is licensed under CC BY-SA 2.0
The cone is to slow down the supersonic airflow, so that the air is relatively calm when it goes into the turbine, which ensures a smooth burn. It's in the nose so that the air duct is straight, because, without computer modelling it took a lot of trial and error to design curved air intakes (e.g. in the wings).

?? I'm a longtime Lurker on this Stack who usually just posts on Stackoverflow, so this requirement is new to me. I got it off Wikipedia, what should I put? Wikipedia, or Whoever it's attributed to there?

A link back to the source page is usually enough. If the image is copyrighted then moderators can easily check the link. If it's wikipedia it's usually enough to just say so (maybe in brackets) below the image.

To be extremely simple without going into too much technical design theroy.
It allows the engine to breath at high speeds.
You will notice that in front of a high speed fan or a cooler it will be extremely difficult for you to breath, try putting your head out of the car and breath when travelling at 60mph , it will be much more difficult to breath compared to doing the same while travelling at 20mph.
Its the same principle, since you cannot reduce the speed of the aircraft to allow it to breath, the cone acts as a hand in front of your nose at 60mph which will allow you to breath and maintain the speed.
It disrupts the direct flow of the air into the engine and reduces the speed of the air to sub sonic levels which the engine can process.
Fun Fact: The design is already present in nature and was lifted off of the peregrine Falcon.

[Head shot by Keven Law protected under CC BY-SA 2.0]

Please provide proper attribution of the image as required by SE

This is a superficial resemblance. This 'unique' (which is not unique at all, as everyone pointed out) sharp cone is only required for supersonic flight. Even the fastest falcons are not supersonic :) The sharp point (or sharp flat edge on most modern designs) creates a slant shockwave (or a series of them: usually there are multiple edges if you look closely), which is better for drag than just an abrupt straight shock.

@Zeus Yes you are absolutely right.. Its only helpful for supersonic speed but that is not what we are comparing here.. The jet engine suffers at supersonic speed , the organic lung suffers at 50 to 100mph be it for a falcon or a human being so the question of sub or supersonic just depends on the engine. We here are talking about the intake.. :-)

I'm not an aircraft designer, but I'm pretty sure I've figured out the design philosophy that led to that nose.  The aircraft has one engine, which is positioned along the aircraft's central axis.  Since it's a jet engine, it needs an intake.  The simplest intake design is a straight tube.  Due to the positioning of the engine, this puts the intake's inlet right at the nose.  As others have mentioned, the cone in the center of the inlet is present in the MiG-21 but not, for example, the F-86 Sabre, because the MiG-21 is a supersonic aircraft, and the F-86 isn't.

There are plenty of single-engine aircraft which do not have a single, central intake. Take for example the F-35 and the Harrier which have their intakes on the sides of the cockpit/fuselage.

Well then perhaps the question should be "what is the design philosophy behind the shaping of the nose of the F-35" or "... the Harrier" or "... the F-16" etc.  Straight-in thru the center of the nose seems so obvious as to require no explicit philosophy.

@A.I.Breveleri The phrase "design philosophy" is neither meaningful nor helpful. It's not about "philosophy"; it's about fluid dynamics and making the thing work.

@SMSvonderTann Yes, but those ducts need to be curved, which is harder to get right.  At the time, supersonic flight was still relatively new, so just making the MiG-21go that fast without breaking up or losing control was enough of a challenge; fancy intake ducting would likely have been too much of a risk.

The Super 7, an updated J-7, the Chinese MiG-21, had a new nose and intakes. Basically, an F-20 Tigershark grafted onto the fuselage of a J-7 (Grumman were consultants on the project). As such, you can, in effect, see a MiG-21 with a more modern nose.

Is there any source for this information that you can link to? On its face, the idea of Grumman consulting with the P.R.C. on a defense contract seems highly unlikely.

@RalphJ It might seem crazy now, but in the 1980s the Chinese were trying to be friends with the west. Grumman were consultants on the Super-7 project. However, after Tiananman Square they were ordered to leave. There are many internet references to it, try 'Super-7 Grumman, and even that some parts of the Super-7 were to be American supplied (avionics anyway).

As mentioned above, it's an effective design solution for the placement of an antenna for a fire control radar and supersonic diffuser for a jet intake all in one package which minimizes the frontal cross section of the aircraft, reducing drag.  As the BVR SARH and TARH missiles became more ubiquitous as air to air weapons, this necessitated larger and larger antenna for the fire control radar set to increase the range in which the radar could acquire and track targets, forcing the placement of engine intakes elsewhere in the aircraft to maximize available real estate on the nose of the aircraft for for these FCRs.

The inception of forward looking radar created a need for a nosecone on aircraft that already had a centered inlet. As radar size and complexity evolved, so did the requirement of an integral nose cone. This also let to the design of the single or split inlets.

The nose cone or spike is designed to take supersonic air and slow it down to subsonic so the jet engine can ingest it. The mig-21 spike moves forward and aft based on speed much like the SR-71 ,except they work opposite. The SR-71 spike starts forward and moves aft as the plane goes faster to bring the shockwave inside the inlet. The mig-21 spike starts aft and moves forward to keep the shockwave outside of the inlet as the plane goes faster because it is a external compression inlet. The Convair B-58 Hustler was the same way. Most supersonic planes including the Concorde have external compression inlets which limit their speed to Mach 2. They have the ability to go faster but above Mach 2 it burns way to much fuel to be efficient.

One point not mentioned here, but crucial to this discussion, is the inescapable fact that, all other factors, i.e. sophistication of technology,"Moore's Law", etc., being considered, the effectiveness of a radar, not unlike the lens of a camera, is determined by the sheer physical size of the antenna, with performance actually increasing EXPONENTIALLY(!!!), the greater the diameter.  Which is why both the MiG-29, and the Sukhoi Su-27/35 series all have such large, bulbous noses, the better to make up for a deficiency in chip design, and thus general electronic performance and capability.  Needless to say, with it being a most basic law of physics, that two objects cannot occupy the same space at the same time, the larger the antenna, and its accompanying radome, the less space available, circumfirentially speaking, for air delivery to the engine, whose needs in this regard, cannot be compromised. Accordingly, and with BOTH(!!) requirements increasing, somebody had to move, and, with the radar antenna having, for obvious reasons, "first dibs" on the nose of the aircraft, the inlets had to "re-locate" to to the flanks. ala, F-102, F-104, F-105, F-106, and F-4, F-111, F-114, F-15, etc., not to mention Migs "23" to "31"! I could say "Lightning" replaced by "Typhoon", or "Mirage" by "Rafale", but you get the point.

This doesn't have anything to do with Moore's Law; we're not dealing with the size of the avionics package but the required detection range of the radar, which dictates antenna size and how much nose real estate it occupies.


text:
What are the physical/aerodynamic implications of designing a prop guard for multirotors?
A number of drones these days like the one shown in the picture below have  fully protected multi-rotor propeller guard system . These are lightweight, aerodynamic housing that keeps these spinning knife-blades away from people's fingers, trees etc.
So,what are the  physical/aerodynamic implications of designing a prop guard for a multirotor drone whose specs are as follows:

All Up Weight with battery: 1050g
Frame weight empty (no electronics, motors, or battery): 400g
Flight time with 3s 2200mah lipo: 8 minutes
Propeller size : 10"
Skeletal Plastic Structure

Source: Safe Flight Copters

Remember the drag contribution of a biplane's bracing wires? Now this quadcopter has not one, but two screens in the flow path of the propellers. Each thread of the mesh will act like a small bracing wire and reduce propeller efficiency considerably.
In comparison, the shroud has relatively little impact. Tip clearances are too big and shroud size is too small to improve propeller efficiency measurably, the drag in forward flight should be manageable and the worst contribution of the shrouds is probably their added mass. But it all pales in comparison to those two layers of mesh!

In general, having a shroud (a duct actually) over the propeller reduces the tip losses; however, in an UAV, this effect is pretty small (and requires tight clearances) in order to have any practical effect. Also, if the ducts are not designed properly, the flow entering the propellers may be affected, reducing the performance.
The main disadvantage is the weight- the shroud and the support structure will affect the performance adversely (also, there is some increased drag) and is usually not recommended. For an UAV having endurance of mere minutes, the effects can be substantial.
The main advantage is safety- the propellers are safe from hitting the ground and getting damaged; so if you are reasonably confident of your flying abilities it is better to go without a shroud, rather than with one.
Another thing is that the addition of aftermarket shrouds to UAVs have to be evaluated carefully- the weight may alter the flying properties appreciably (for e.g. the descent rate), so one has to be careful.

Aerodynamically, one can say that those prop guards create some kind of Fenestron (Like some tail rotors in helicopters), which will reduce the tip losses of the propeller.
On the other hand, they will increase drag in forward flight, reducing the UAV top horizontal speed. If you take a look at the Aibotix X6 you see a one piece prop guard designed aerodynamically to reduce drag in forward flight.
In the particular case of your image, the black web also decreases the area of air flow into the propeller, certainly reducing its efficiency.
Another prop guards disadvantage is the weight they add. Even though they might be made of lightweight material, they have to have some strength to resist any impact.
To sum up, with a prop guards like those, you should loose top speed and around 2 minutes of flight time.
One last tip: if you want to fly your UAV close to trees, forests or power lines, be careful with the prop guards because you might end with your UAV hanging on a tree branch 10m high... (happened to me :/)


text:
How do UAV designers improve visual quality without compromising computational efficiency in mini UAVs?
I'm designing an Unmanned Aerial Vehicle(UAV) similar in size to the RQ 11B Raven.

This UAV can carry color and infrared cameras to capture outside scenes in real-time. The quality of imagery that these UAVs relay to the operators is directly related to how much of information they can extract from the frames being captured in real-time. So far the captured video looks jittery and not appealing due to vibrations of the platform. I am encountering two issues here

issue of oscillations ; and
low frequency vibrations

Can you guys please suggest how UAV designers have managed to address these 2 issues to improve visual quality without compromising computational efficiency?

Mounting the cameras on shock-absorbing mounts (rubber bushings). Make sure the prop/motor is balanced.

The problem is commonly referred to as "jello". Here's an article about it: http://dronetechreport.com/reducing-jello-getting-natural-look-aerial-video/

@Steve: Thank You. I know the effect is called Jello in some circles however none of the materials I have come across explains how UAV designers do not compromise on computational efficiency while addressing the issues of vibration & oscillation. Large UAVs like Predator have a lot of computational power so these issues are not major issues, however for mini/micro UAVs striking a balance is extremely difficult.

I'm not sure why computational efficiency is a problem, as this can be solved by mechanical means.

Yes, I realise it is possible. That's my question - the how part.

Given that the Raven you posted costs $173,000 per unit, I would say that they use some pretty advanced electronics, hardware, and software to solve this problem. Not something I think you'll be able to duplicate for a reasonable amount of money. I'm not sure this is entirely on topic here, as you are asking about image stabilization which isn't necessarily the best place to ask...

Would it be possible to post a snippet of video? It's a little unclear what you mean by oscillations and how low is low frequency. You might also indicate if your camera is shock mounted and if so how

I'm not sure why computational efficiency is a concern for vibrations. They're very predictable, a Raspberry Pi Compute Module probably is sufficient (7.2 grams).

First look at the frequencies involved. If your camera frame shakes so badly that the picture quality suffers visibly, I would suspect that your camera mount has an eigenfrequency close to whatever causes the vibrations (the engine, most likely). Maybe it is enough to shift the eigenfrequency of the camera mount away, either by changing the mass of the camera or by changing the stiffness of the mount.
It could also be that some structure other than the camera mount is in frequency neighbourhood with the engine and the camera is mounted near an antinode - maybe it is enough to shift the camera location a little.
Vibration problems are hard to diagnose from the little information you give, so all I can do is to guess the most likely cause of your troubles.

First, The low frequency dynamics that you mention probably comes from the movement of the aircraft, due to wind, gust, etc. one idea is to have a neutrally stable aircraft in pitching motion due to airspeed. This is not always possible but may help a bit.
Second, the high frequency distortion (shake) most probably is poor damping / installation.
Thirdly, to get crisp images cameras are built into gimbals, that actively suppress unwanted motions.
Lastly, postprocess video enhancement is possible, but for your application, probably this is not desirable.
And of course some cameras have video stabilization capability. That should solve half the problem.


text:
How is Spitfire landing gear powered?
I have seen a WW2 movie where the pilot of the Supermarine Spitfire has difficulties to extend the landing gear as the engine is no longer running.
This would be reasonable if the gear normally needs hydraulic or electric power to be extended, otherwise probably just an attempt to add more tension to the plot. Would running out of fuel (no engine power) make the extension of the Supermarine Spitfire landing gear more difficult or different from usual?

From the manual looks like it was hydraulic power with CO2 cylinder backup. Ok this is the answer. That interceptor is more complex than I assumed.

Early models of Supermarine Spitfire (prior to the 175th production aircraft) had no hydraulic gear extension system, according to Wikipedia:

At the same time the manual hand-pump for operating the undercarriage
  was replaced by a hydraulic system driven by a pump mounted in the
  engine bay

Source
The movie could show the normal extension of the undercarriage, using a lever, but a lost engine would not impact the operation.

After the 175th aircraft, the gear was hydraulics powered, so without engine running, the pressure could be insufficient. The design included an emergency extension using a $\small \textrm {CO}_2$ cylinder:

Source: 1940 Spitfire Manual
The system is visible on this image (black bottle with red handle "Emergency Only"):

Source
The hydraulic gear control box is at the center.

So it would appear that Dunkirk got this bit slightly wrong, as there is a scene where a pilot pumps a handle up and down repeatedly to lower the landing gear, violating the instructions in the last sentence of paragraph (b).

Chassis control handle - how very British.....

Paragraph (b) refers to later Spitfires with the CO2 emergency system. If the Spitfire in the movie was an earlier version, the landing gear would have to be pumped down. "the manual hand-pump for operating the undercarriage was replaced by a hydraulic system driven by a pump mounted in the engine bay"

@mins, Yes, I was only responding to "David K".  "Dunkirk" was correct providing it was an early Spitfire.  There are so many other things wrong with the movie, I highly doubt they researched it well enough to fully understand variations in the workings of the Spitfire undercarriage systems.


text:
Does Karman line depend on the wing area, and if not, why?
The Karman line is the altitude at that the airspeed required to generate the sufficient lift to support itself exceeds the orbital speed. Looks like the Karman line is defined as some fixed value, equal to 100 Km. How this could possibly be?
In a usual flight, "the airspeed required to support itself" depends dramatically on the wing and can vary I think from 29 km/h for Gossamer Albatross till 389 km/h for X-15.

The calculation definitely does depend on wing loading. However, the Kármán line does not, because it is not actually defined by that calculation, it is defined by statement. The calculation, for a “typical” airplane, is just motivation for the value.

This answer is partly based on:

FAI, The Kármán line definition

Nicolas Bérend, The Missing Calculation behind the Original Kármán Line Definition.

Thomas Gangale, The Non-Kármán line: An urban legend of the space age.

Summary
To directly answer the wing area question: The wing area cannot be increased without increasing the aircraft mass. Both lift and centrifugal force are increased, but their ratio won't change significantly. Kármán line altitude is more or less based on the ratio. But the main point is the Kármán line altitude of 100 km wasn't the result of a calculation (and it wasn't proposed by Kármán!).
The idea of a limit for atmospheric flight was presented in a 1956 paper by Andrew Haley with an earlier diagram from Masson and Gazley. While Masson and Gazley were interested in aircraft skin heating, Haley was only trying to define a legal value for space delineation. Masson and Gazley's diagram shows a "lift barrier" where the increased velocity required due to the low air density creates more centrifugal force than lift.
This is of prime interest since when the aircraft is more orbiting than flying, it is a spacecraft no more concerned by aircraft regulations. The barrier has a steep segment starting at 275k ft (83.8 km), this is where the atmosphere-space transition occurs.
Kármán too reused the diagram in one of his own paper and mentioned that for legal purposes, space could be seen as beginning near 300k-400k ft (91 to 122 km). The nickname Kármán line was coined by Andrew Haley and was initially referring to Masson and Gazley 275k ft:

a critical jurisdictional line, marking the theoretical limit of air flight, which I term the Kármán Line. It must be noted with care that the exact location of this line of primary jurisdiction is not presented as an apodictic solution of the problem. The Kármán primary jurisdictional line may eventually remain, or, after due consideration of such developments as improved techniques of cooling and the discovery of more heat resistant materials, this line may be changed significantly. But, while these changes will be in the exact location of the Kármán Line, the existence of the line is certain and wherever the line is finally drawn will be the place where “airspace” terminates.

Source.
The rounded 100 km value was set by FAI for air competition purposes, likely inspired by Kármán's range and Haley's nickname.
The 1956 diagram was built based on a lift parameter, mixing wing loading and lift coefficient, of 80 psf (381 kg/m²), but the aircraft configuration has actually little impact on the altitude, as at this altitude nearly no lift is generated.
A scientific delineation of space exists, based on the maximum equilibrium velocity. Taking the same value for the lift parameter, the calculated limit is 110 km, varying the parameter has little influence on this figure.

Details
This all started with this 1956 diagram:

Source.
published in a paper by Masson and Gazley: Surface Protection And Cooling Systems For High-Speed Flight. This paper was related to aircraft materials, not to the limit of atmosphere.
About the diagram
Level flight at a certain (density) altitude requires a certain velocity to create the desired amount of lift. In turn velocity and air density create a certain amount of friction heating the aircraft skin. This diagram divides the velocity-altitude domain into three parts:

The bottom-right corner corresponds to skin temperature exceeding 2000°F (1093°C), the limit assumed for skin materials.

The top-left corner corresponds to insufficient lift to maintain a level flight, based on $\frac M {S.C_L} $ = 80 psf (381 kg/m²). According to Bérend: The curve and the associated 80 psf value actually comes from a 1953 article by Fritz Haber.

The central corridor corresponds to a region were flight is possible.

An aircraft produces lift but, because Earth is a spheroid, it also produces a centrifugal force. As altitude increases, level flight requires a higher velocity, but air density decreases, the ratio lift/centrifugal force actually decreases. At some point only the centrifugal force contributes to maintaining the constant altitude. The aircraft is not flying anymore (aerodynamic force) but orbiting (micro-gravity from orbital velocity).
As seen on the diagram, there is a sudden change in the ratio lift/centrifugal force near 275k ft. At this altitude increasing airspeed has nearly no influence on lift. The aircraft aerodynamic characteristics play no role.
Haley's paper
The diagram was reproduced by Andrew Haley in a 1957 paper: Space Law, The Development of Jurisdictional Concept. Haley, discussed the diagram with Kármán, and proposed to set the limit of atmospheric flight somewhere after 83.8 km (275k ft). At this altitude the required velocity to maintain a circular orbit is about 28,290 km/h (25,780 ft/s). Haley insisted on the fact such limit wasn't a scientific matter, but a legal one.
Kármán's proposal and FAI decision
In his conferences on astronautics Kármán consistently proposed to set the beginning of space at a rounded number between 300k and 400k ft (91 to 122 km). In turn the FAI, fédération aéronautique internationale, set the limit for flight at 100 km. This value is by no mean calculated, but based on Gazley's diagram.
Actual calculated Maximum Equilibrium Velocity
A scientific comparison of aeronautical/orbital effects sets a maximum equilibrium velocity (MEV) altitude at 110 km, another rounded value, calculated with the same lift parameter of 80 psf (381 kg/m²):

Source.
This value again doesn't vary much with aerodynamic characteristics:

~112 km for ~200 kg/m²
~105 km for ~800 kg/m²

Why does the reducing density near 275k ft break the usual lift-airspeed relationship?  Too few molecules hitting each other to form the usual pressure distribution around a wing?

@CamilleGoudeseune: The break happens when you hit 25k ft/sec – orbital velocity. From this point onward you don't need aerodynamic lift at all. You can suddenly go as high as you want. That's why the curve turns vertical.

potentially interesting (and potentially further answerable) in Space SE: What would a "Kármán plane" look like, a bird, or a plane?

@RainerP that's reversing cause and effect.  I'm curious why lower density breaks "lift ∝ v^2."  Maybe I should ask on Physics SE.  Maybe something about "collisional atmosphere."

@RainerP. - orbital velocity cares nothing about the atmosphere, it depends only on the distance from the center of mass.  You could "hit" orbital velocity at 150,000 feet, however you would need good heat shielding and also have to maintain continuous rocket thrust to maintain orbit. There is a common misconception that at the Karman Line you can cut your engines and orbit, but you can't, as none of the "Karman" calculations are concerned with how much of your forward motion comes from momentum and how much is from rocket thrust. Minimum orbital altitude with engines shut off is about 200 km.

The forbidden region in the diagram is weight > lift + centrifugal. At a speed of 20k ft/s, you need some lift and that limits you to 230k ft altitude. If you accelerate to 25k ft/s, you still need a little aerodynamic lift and that limits you to 270k ft altitude. If you accelerate to 26k ft/s, then weight > lift + centrifugal is never true because the centrifugal force alone is already greater than weight. At this point, the forbidden region ceases to exist. That's what causes the knee in the diagram. I don't see any evidence that aerodynamic lift changes significantly at 275k ft.

Mathematically, the curve is something like 10-log((1-v²)/v²) where 1 is weight and 1-v² is weight minus centrifugal force, aka need for lift. We divide that by v² (lift also goes with v²) to get need for air density. We then take the logarithm to convert density to altitude. The curve looks pretty close, and I used plain v² relationships for centrifugal force and aerodynamic lift without any sudden change at 275k ft. I still get the knee.

@RainerP.: Haley published an augmented version of the original diagram (source) which may provide more insight.

@RainerP. Is there any chance that this knee is related to that knee? Why does Earth's atmospheric density have a big "knee" around 100 km? Is there a good analytical approximation? I also wonder how Karman actually felt sometimes: Has there ever been a case where someone wished a theorem or important result wasn't named after them? Has it happened more than once?

@uhoh: no, i was referring to the knee where the line sharply bends upwards and asymptotically approaches a vertical line defined by orbital velocity. (you can clearly see that in mins' augmented diagram). There was some debate in the comments about sudden changes in atmospheric conditions at 275k ft causing the knee, and I was trying to refute that.

From the Wikipedia article:

Although the calculated altitude was not exactly 100 km, Kármán proposed that 100 km be the designated boundary to space, because the round number is more memorable, and the calculated altitude varies minutely as certain parameters are varied.

(emphasis mine)

The difference is not 300 km/h, the difference is 13×, and stays so as the density decreases, because the density is multiplicative factor (true airspeed increases inversely with density; update: I first said stall speed, but since this is supersonic, the flow is never attached in the first place).

The WP statement has been changed meanwhile. It's actually not sure whether Von Kármán himself proposed the 100-km-value. His calculations were in the 80~90 km range.

100 km was not proposed by Kármán, but set by the FAI, based on Kármán idea it should be a rounded number between 300 and 400k ft.

Also the FAI aren't necessarily the experts on space, they started out over 100 years ago certifying balloon records, and then expanded into aviation records, and then sort of inserted themselves into space records and setting the definition for space. Which is fine, there needs to be some coordination, but many people think the FAI is authoritative on the scientific subject of where space begins


text:
How high does the ground effect last?
Up to which height is the ground effect still significant enough to be taken into account? Would this height be sufficient to actually fly above the ground, overcoming at least trivial obstacles like trees?
The early transatlantic flights were done with heavy loaded planes that had difficulties taking off (only at the end of the runway, and only just). Could it be that these planes were flying within ground effect for rather extended distances, before picking enough speed to complete the take off?

I believe it is roughly half the wingspan.

You mean something like a Ground Effect Vehicle except also being able to fly (a Type-C or Type-B)? Usually one is designed to be one or the other, but not both. Lindbergh flew at low altitudes but was quite a bit higher than being in ground effect.

I lost a cylinder in a Cessna 150 just as I rotated for takeoff at 5000'+. Got over low wires but was on the ragged edge as to the backside of the power curve. Any bank would put me right down on the deck. I'm sure circumstances vary but my experience was just on the edge of flying.

Ground effect height is primarily a function of the airfoil, and the "ground" surface. Smooth desert lakebeds have "better" ground effect than forest tree tops. Calm water is better than open water for smoothness.
As I recall, there have been several times where planes have operated in ground effect for substantial distances, such as military aircraft loosing engines at sea.
So to the OP questions:

The general rule is that 1/2 the wingspan is the practical limit of ground effect. My spin is that the surface of the ground, and the airfoil, vortex and downwash factor into that distance.
Obstacles such as trees are not practically overcome by ground effect.
Numerous documented instances have historically occurred where aircraft have extended operations over large bodies of water with engines out, or lower power settings, using ground effect.  As fuel is burned off, sometimes the ceiling of the aircraft will increase notably.

There is no height at which ground effect is switched off. It just becomes less and less effective. There are two factors which determine how strong ground effect is.:

Height relative to wingspan: The downwash cannot move into the ground so the induced flow field around the wing is distorted. In a first order approximation, the wing affects mostly the air which flows through a stream tube with the diameter of the wing span. For an explanation see this answer. Once the height is above one semispan, the distortion of the flowfield comes to an end.
Trailing edge height relative to wing chord. For very small heights, the gap between trailing edge and ground becomes small enough to more or less block the proper exit of the flow below the wing. The result is an increase in ram pressure on the lower side and further distortion of the flow field such that the stagnation point moves down and the suction peak of the nose flow becomes stronger, adding nose thrust and reducing drag.

When moving up from zero height, ground effect diminishes rapidly because the gap between trailing edge and ground opens up and the ram effect vanishes. Moving up further will also weaken the first effect until at a height of one semispan the flow will be very similar to that of the airplane out of ground effect.
To illustrate how much happens on the last centimeters (or feet), look at the two pictures which I took from this YouTube video:

In the top picture the wing is clearly closer to the ground than its semispan, but the vortices from the flap edge trail straight behind the plane. Now move forward to the lower picture when the trailing edge is maybe half a chord above the ground. Now the trailing vortex is swept to the side because the flow, which is compressed below the wing, expands when it exits the gap between wing and ground. Additionally, the downwash becomes a sidewash, because it has no space below the wing left in which to drop.
This should demonstrate that most of the ground effect happens when the ram pressure below the wing builds up. Once the trailing edge is less than half the wing chord from the ground, this part of ground effect kicks in, and the aircraft sees a significant reduction in drag.

I remember studying that ground effect happens within one wingspan. Although it reduces when you gain altitude.
As mentioned in FAA’s Pilot’s Handbook of Aeronautical Knowledge:

When the wing is at a height equal to its span, the reduction in
  induced drag is only 1.4 percent. However, when the wing is at a
  height equal to one-fourth its span, the reduction in induced drag is
  23.5 percent and, when the wing is at a height equal to one-tenth its span, the reduction in induced drag is 47.6 percent.

It is also referenced here:

Ground effect only begins to show up when you're within one wingspan
  of the ground. But, it really reduces your drag when you're within 20%
  of your wingspan to the ground. At that height, your wing only
  generates 60% of its normal induced drag.

Although it depends of the characteristics of the wings (shape, wingspan,...), as you said it can be useful to gain some lift. I would say from 5 to 10 meters (for a medium sized plane), the effect loses a lot of its efficiency. Unfortunately, I have no mathematical proof to back this up.
I don't know if you ever heard about them, but ekranoplans use almost exclusively the ground effect to "fly", however, they can't gain altitude like an actual plane would do. As @mongo stated, smooth surfaces are much better to take advantage of this effect.
More examples of Ground effect vehicles here

When the first ekranoplans were spotted on surveillance pictures, US intelligence was very worried, because nobody could figure out how such a monstrous aircraft with such stubby wings could ever fly, and they feared that the Russians might have discovered some completely new physics that the US didn't have. Of course, the solution to the riddle was much simpler: the US physicists were right, they can't fly, they can only skim the surface in ground effect, and do the occasional "hop".

Yes, I watched a very interesting documentary on Cold War inventions and the US was very worried about this Caspian Sea Monster. In fact, it was more a boat than a plane in terms of military usage.


text:
Can two captains fly an airliner?
If the First Officer cannot take his role for some reason, and the company only has another Captain available, can two captains fly the airliner, assuming they routinely fly as captains for that company in the aircraft of that type? Can one of them simply take a FO role for that flight?
In other words, is the First Officer a separate certification that must be current?

Possible duplicate of Can a captain fly in the right seat as SIC under part 129?

This plane isn't big enough for the both of us.

Yes.
What allows you as a pilot to fly large aircraft (legally under the FAA) is a type rating. As long as the two Captains hold proper type ratings for the aircraft they suffice for the required two crew members needed to fly the plane. However one of the two of them will be performing the duties of the first officer and that should be decided prior to the flight.
First Officer and Captain are rankings given by the airline not any governing body. Both the FO and Captain cary the type cert required to fly the aircraft (as far as I know).
The limited (and quickly dwindling) exception to this is aircraft that require a crew of 3 where the third is a flight engineer. The Flight Engineer position does hold its own certification.

And it isn't unheard of for the person in the right seat to have more seniority, "rank", and experience than the Captain in the left seat.

Two-captain pairings are possible, and not particularly exceptional even -- obviously, both captains are fully type rated in the aircraft, so together they make a full crew for the aircraft (barring a F/E as Dave points out).
There are two scenarios where this is common:

A pilot is being upgraded to Captain of a specific type or is otherwise undergoing training "on the line", such as differences training.  He will occupy the left seat, while the instructor Captain will occupy the right seat during his line upgrade flying.
A line check is being performed.  In this case, the line captain occupies the left seat, while the check captain occupies the right seat.

There is also the case where while you have a captain and a first officer, the first officer has more seniority/experience than the captain.  This is common with a junior captain -- "green on green" rules prohibit pairing a junior captain with a junior first officer, so many times, a junior captain will fly with senior first officers until they get some seasoning of their own under their belt.

I would presume that in the first case the 'instructor Captain' is actually PIC (or 'captain' as the older terminology would have it).

@DJClayworth -- I believe that's true, yes

Yes if the captain is right seat checked - in my oufit anyway. At my previous employer all capatain were approved, in my current one just management and trainers. Makes for easier scheduling perhaps but requires a bit more sim time for the right seat V1 cut check.

What do "right seat checked" and "right seat V1 cut check" mean?

the reccurent training needs to be done for a specific (right/left) seat. If the captain did not have a check in the last 6 months in the right seat them he/she cannot legaly fly from the right. It will cost slghtly more money to train all captains for both seats